Experience in the laboratory of the Center PI has demonstrated the improved efficiency, cost-saving, and success of centralizing ES cell technology. A common source of ES cells, tissue culture reagents, and methods will be used in the knock-out experiments under the supervision of experienced individuals. The PI (S.H. Orkin) will direct the ES tissue culture-mouse core and delegate responsibility to two individuals, Dr. Fong-Ying Tsai and Ms. Margie Rosenblatt (see Project I). Both are experienced in all aspects of ES cell culture and generation of germline mouse chimeras. By centralizing reagents, methods, and technology we believe that problems encountered in establishing the ES system de novo in a laboratory for purposes of a knock-out project will be eliminated. Over the tenure of this Center we envision that the ES tissue culture-mouse core will be increasingly used as a resource to facilitate collaborations with other investigators in the Harvard Medical area. Core A will provide ES cell, tissue culture reagents, and advice for the isolation of gene targeted clones. Once gene targeted clones are identified, the core will provide for injection of the clones into mouse blastocysts for the generation of chimeras, test breed chimeras for germline transmission, and breed heterozygotes to homozygosity.